Use the method "addAuthorizationRole" to add the roles with same name successfully

I called the method  "addAuthorizationRole" to add a role named "testRole" twice. In the vCenter Server 6.0, "AlreadyExists" fault occured. but in the vCenter Server 5.1&5.5, I can add  the roles with same name successfully, is it a bug of vCenter Server 5.1&5.5?